A carbon nanotube (CNT), which is a kind of carbon allotrope, is a material having a diameter of ones to tens of nm and a length ranging from hundreds of μm to ones of mm, and has been studied for use in various fields due to its excellent thermal, electrical, and physical properties and high aspect ratio since it was reported in the journal Nature in 1991 by Dr. Iijima. Such carbon nanotubes have inherent properties caused by the sp2 bond of carbon, are stronger than iron and are lighter than aluminum, and exhibit electrical conductivity similar to metal. Carbon nanotubes are largely classified into, depending on the number of walls of nanotubes, single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWNTs), and multi-wall carbon nanotubes (MWNTs), and have zigzag, armchair, and chiral conformations depending on their asymmetry and chirality.
A carbon nanotube (CNT) fiber is manufactured through forest spinning or direct spinning. The forest-spinning process is performed in a manner in which a catalyst is deposited on a substrate, a CNT forest is synthesized in a direction perpendicular to the substrate, and CNTs at the ends of the substrate are pulled with tweezers or tape, whereby the CNTs are connected due to van der Waals force between CNTs, thus spinning a CNT fiber. This method is disadvantageous because a continuous process cannot be implemented, making it impossible to increase the productivity thereof.
Meanwhile, a carbon nanotube (CNT) fiber includes various kinds of impurities. In particular, the impurities that are the most abundantly present are amorphous carbon, which is decomposed in a gas phase and formed inside and outside the fiber. Such impurities degrade the quality of the fiber and must be removed.
Typically, amorphous carbon may be removed through oxidation at a temperature of 400 to 500° C. The removal of amorphous carbon is currently performed by passing a fiber through a heat source such as a furnace. However, this method negates economic benefits because an expensive furnace has to be used. Moreover, since the furnace has a limit as to its length, a plurality of furnaces has to be provided to increase the retention time of the fiber, thus negating economic benefits. Alternatively, the rate of movement of a fiber passing through a reactor may be decreased, which may also undesirably reduce productivity. Hence, a novel method of removing impurities is required.